Ions and ionic bonding

What you'll learn

This topic examines how atoms gain or lose electrons to form charged particles and how these particles bond together to create ionic compounds. Understanding ions and ionic bonding is essential for CIE IGCSE Chemistry papers, appearing regularly in both multiple-choice and structured questions worth 6-10 marks per paper.

Key terms and definitions

Ion — a charged particle formed when an atom loses or gains electrons, resulting in unequal numbers of protons and electrons.

Cation — a positively charged ion formed when an atom loses one or more electrons; metal atoms typically form cations.

Anion — a negatively charged ion formed when an atom gains one or more electrons; non-metal atoms typically form anions.

Ionic bond — the strong electrostatic force of attraction between oppositely charged ions in an ionic compound.

Electrostatic attraction — the force between opposite charges that holds ions together in a lattice structure.

Giant ionic lattice — a three-dimensional repeating structure of ions held together by ionic bonds extending in all directions.

Electron configuration — the arrangement of electrons in shells (energy levels) around the nucleus of an atom or ion.

Oxidation — the process of losing electrons; occurs when atoms form positive ions.

Core concepts

Formation of ions from atoms

Atoms form ions to achieve a stable electron configuration, typically matching the nearest noble gas. Metal atoms lose electrons from their outer shell, while non-metal atoms gain electrons to complete their outer shell.

Group 1 metals (lithium, sodium, potassium) lose one electron to form 1+ ions:

• Na (2,8,1) → Na⁺ (2,8) + e⁻
• The sodium ion has the same electron configuration as neon

Group 2 metals (magnesium, calcium) lose two electrons to form 2+ ions:

• Mg (2,8,2) → Mg²⁺ (2,8) + 2e⁻
• The magnesium ion has the same electron configuration as neon

Group 3 metals (aluminium) lose three electrons to form 3+ ions:

• Al (2,8,3) → Al³⁺ (2,8) + 3e⁻

Group 7 non-metals (fluorine, chlorine, bromine, iodine) gain one electron to form 1- ions:

• Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8)
• The chloride ion has the same electron configuration as argon

Group 6 non-metals (oxygen, sulfur) gain two electrons to form 2- ions:

• O (2,6) + 2e⁻ → O²⁻ (2,8)
• The oxide ion has the same electron configuration as neon

Transition metals can form ions with different charges (e.g., Fe²⁺ and Fe³⁺, Cu⁺ and Cu²⁺). The charge must be specified when naming these compounds.

How ionic bonds form

Ionic bonding occurs through electron transfer between a metal atom and a non-metal atom. The metal atom loses electrons to form a positive ion, while the non-metal atom gains these electrons to form a negative ion. The oppositely charged ions are then held together by strong electrostatic forces.

Step-by-step process for sodium chloride (NaCl):

1. Sodium atom (2,8,1) loses one electron from its outer shell
2. Sodium becomes Na⁺ ion (2,8) with a 1+ charge
3. Chlorine atom (2,8,7) gains one electron into its outer shell
4. Chlorine becomes Cl⁻ ion (2,8,8) with a 1- charge
5. The Na⁺ and Cl⁻ ions are attracted by electrostatic forces
6. Many ions arrange themselves into a giant ionic lattice

For magnesium oxide (MgO):

1. Magnesium atom (2,8,2) loses two electrons
2. Oxygen atom (2,6) gains two electrons
3. Mg²⁺ and O²⁻ ions form
4. Strong ionic bonds hold the ions in a lattice structure

For magnesium chloride (MgCl₂):

1. One magnesium atom loses two electrons to form Mg²⁺
2. Two chlorine atoms each gain one electron to form two Cl⁻ ions
3. The ratio is 1:2 to balance the charges (1 × 2+ = 2 × 1-)

Dot-and-cross diagrams for ionic compounds

Dot-and-cross diagrams show the electron transfer and final electron arrangements in ions. Electrons from one atom are represented as dots, while electrons from the other atom are shown as crosses.

Key rules for drawing ionic dot-and-cross diagrams:

• Show the electron configuration of the original atoms
• Use dots for electrons from one element, crosses for the other
• Draw square brackets around each ion
• Write the charge outside the bracket
• Only show outer shell electrons in the ions (for CIE IGCSE level)

For sodium chloride:

• Na starts with one electron in its outer shell (shown as a dot)
• Cl starts with seven electrons in its outer shell (shown as crosses)
• After transfer, Na⁺ has lost its outer shell entirely
• Cl⁻ has eight electrons (seven crosses plus one dot) in its outer shell

For magnesium oxide:

• Mg transfers both outer electrons (shown as dots) to oxygen
• O²⁻ has eight electrons in its outer shell (six crosses, two dots)
• Both ions achieve stable noble gas configurations

Properties of ionic compounds

The giant ionic lattice structure determines the characteristic properties of ionic compounds tested in CIE IGCSE Chemistry examinations.

High melting and boiling points:

• Ionic compounds have melting points typically above 500°C
• Strong electrostatic forces between oppositely charged ions require large amounts of energy to break
• The forces extend throughout the entire lattice in all directions
• Example: sodium chloride melts at 801°C; magnesium oxide at 2852°C

Electrical conductivity:

• Solid ionic compounds do NOT conduct electricity because ions are fixed in position and cannot move
• When melted (molten state), ionic compounds conduct electricity because ions are free to move and carry charge
• When dissolved in water (aqueous solution), ionic compounds conduct electricity because ions separate and move freely through the solution
• This property distinguishes ionic compounds from covalent compounds in practical tests

Solubility in water:

• Many ionic compounds dissolve in water (are soluble)
• Water molecules attract and surround the ions, pulling them away from the lattice
• The process is called dissolution
• Not all ionic compounds are soluble (e.g., silver chloride, barium sulfate are insoluble)

Brittleness:

• Ionic crystals shatter when struck
• A strong force can shift one layer of ions relative to another
• Like charges then come next to each other and repel
• The crystal breaks apart along the plane

Writing formulae of ionic compounds

The overall charge on an ionic compound must be zero. The number of each type of ion must balance the total positive and negative charges.

Method for determining formulae:

1. Write the symbols of the ions with their charges
2. Determine the lowest whole number ratio that balances charges
3. Write the formula without showing charges

Examples:

• Sodium chloride: Na⁺ and Cl⁻ combine in ratio 1:1 = NaCl
• Magnesium chloride: Mg²⁺ and Cl⁻ combine in ratio 1:2 = MgCl₂
• Aluminium oxide: Al³⁺ and O²⁻ combine in ratio 2:3 = Al₂O₃ (2 × 3+ = 6+; 3 × 2- = 6-; charges balance)
• Calcium nitrate: Ca²⁺ and NO₃⁻ combine in ratio 1:2 = Ca(NO₃)₂ (brackets used for compound ions when more than one is needed)

Common compound ions to memorise:

• Hydroxide: OH⁻
• Nitrate: NO₃⁻
• Carbonate: CO₃²⁻
• Sulfate: SO₄²⁻
• Ammonium: NH₄⁺

Worked examples

Example 1: Describing ionic bond formation

Question: Describe, in terms of electrons, what happens when magnesium reacts with oxygen to form magnesium oxide. Include the electron configurations of the ions formed. [4 marks]

Answer:

• A magnesium atom loses two electrons [1]
• to form a Mg²⁺ ion with electron configuration 2,8 [1]
• An oxygen atom gains two electrons [1]
• to form an O²⁻ ion with electron configuration 2,8 [1]

Mark scheme notes: Each point is worth one mark. Must specify the number of electrons transferred and the correct charges on the ions. Electron configurations must be given as stated in the question.

Example 2: Explaining electrical conductivity

Question: Sodium chloride conducts electricity when molten but not when solid. Explain why. [3 marks]

Answer:

• In solid sodium chloride, the ions are fixed in position in the lattice [1]
• In molten sodium chloride, the ions are free to move [1]
• Moving ions carry electrical charge through the substance [1]

Mark scheme notes: Must link the ability to conduct to movement of charged particles (ions). Simply stating "ions are present" in both states without discussing movement scores zero marks.

Example 3: Working out ionic formulae

Question: Deduce the formula of aluminium sulfate. [2 marks]

Answer:

• Al³⁺ and SO₄²⁻ ions [1]
• Ratio 2:3 gives Al₂(SO₄)₃ [1]
• (Check: 2 × 3+ = 6+ and 3 × 2- = 6-, charges balance)

Mark scheme notes: One mark for identifying the ions and their charges (may be implied), one mark for the correct formula with brackets properly placed.

Common mistakes and how to avoid them

• Mistake: Confusing the number of electrons with the number of atoms in a formula. For example, writing Mg₂Cl₂ instead of MgCl₂. Correction: The subscript numbers in a formula represent how many atoms or ions of each element are present, not how many electrons are transferred. Balance the charges, not the electrons.

• Mistake: Drawing ionic diagrams showing electrons shared between atoms rather than transferred. Correction: Ionic bonding involves complete electron transfer from metal to non-metal. In dot-and-cross diagrams, draw separate ions in square brackets with charges, not overlapping circles.

• Mistake: Stating that solid ionic compounds conduct electricity because they contain charged particles. Correction: While solid ionic compounds do contain ions, these ions cannot move. Only state that molten or dissolved ionic compounds conduct because the ions are free to move and carry charge.

• Mistake: Forgetting to use brackets around compound ions when more than one is needed, e.g., writing CaNO₃₂ instead of Ca(NO₃)₂. Correction: Always place brackets around a compound ion (polyatomic ion) when the subscript applies to the entire group, not just the last element.

• Mistake: Assuming all metal and non-metal combinations have a 1:1 ratio in their formula. Correction: Always work out the ratio by balancing the charges. Mg²⁺ and Cl⁻ need a 1:2 ratio (MgCl₂), not 1:1.

• Mistake: Describing ionic bonding as "atoms sharing electrons to become stable." Correction: Ionic bonding involves electron transfer (loss and gain), not sharing. Electron sharing occurs in covalent bonding, which is a different topic.

Exam technique for Ions and ionic bonding

• "Describe" questions require you to state what happens in a process. For ion formation, specify which atom loses or gains electrons, how many electrons are transferred, and what ions form with their charges. Two to three marks typically awarded for complete descriptions.

• "Explain" questions demand reasons or mechanisms. When explaining properties like electrical conductivity or high melting points, link the macroscopic property to the structure (giant ionic lattice) and bonding (strong electrostatic forces). Use connecting words like "because" and "therefore."

• Dot-and-cross diagram questions typically award 2-3 marks: one for correct electron arrangements, one for square brackets and charges, one for proper distinction between dots and crosses. Draw neatly and ensure all electrons are shown clearly.

• Formula deduction questions award marks for the correct formula. Show working by writing ion symbols with charges first, then working out the ratio. This allows partial credit if your final formula is incorrect but your method is sound.

Quick revision summary

Ions form when atoms lose or gain electrons to achieve stable electron configurations. Metal atoms lose electrons to form cations; non-metal atoms gain electrons to form anions. Ionic bonds are strong electrostatic attractions between oppositely charged ions arranged in a giant lattice structure. Ionic compounds have high melting points, conduct electricity when molten or dissolved (but not when solid), and often dissolve in water. Write formulae by balancing the charges of the ions. Draw dot-and-cross diagrams showing complete electron transfer with ions in square brackets bearing their charges.